nir: Simplify some comparisons like a+b < a

nir: Simplify some comparisons like a+b < a

All Gen7+ platforms had similar results. (Skylake shown)
total instructions in shared programs: 14514555 -> 14514547 (<.01%)
instructions in affected programs: 1972 -> 1964 (-0.41%)
helped: 8
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.39% max: 0.42% x̄: 0.41% x̃: 0.41%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -0.41% -0.40%
Instructions are helped.

total cycles in shared programs: 533141444 -> 533136780 (<.01%)
cycles in affected programs: 164728 -> 160064 (-2.83%)
helped: 181
HURT: 3
helped stats (abs) min: 2 max: 94 x̄: 26.17 x̃: 30
helped stats (rel) min: 0.12% max: 5.33% x̄: 3.42% x̃: 3.80%
HURT stats (abs)   min: 4 max: 54 x̄: 24.00 x̃: 14
HURT stats (rel)   min: 0.20% max: 2.39% x̄: 1.09% x̃: 0.68%
95% mean confidence interval for cycles value: -27.12 -23.58
95% mean confidence interval for cycles %-change: -3.54% -3.16%
Cycles are helped.

Sandy Bridge
total instructions in shared programs: 10533667 -> 10533539 (<.01%)
instructions in affected programs: 10148 -> 10020 (-1.26%)
helped: 124
HURT: 0
helped stats (abs) min: 1 max: 2 x̄: 1.03 x̃: 1
helped stats (rel) min: 0.39% max: 4.35% x̄: 2.20% x̃: 2.04%
95% mean confidence interval for instructions value: -1.06 -1.00
95% mean confidence interval for instructions %-change: -2.46% -1.95%
Instructions are helped.

total cycles in shared programs: 146136887 -> 146132122 (<.01%)
cycles in affected programs: 206382 -> 201617 (-2.31%)
helped: 171
HURT: 0
helped stats (abs) min: 2 max: 40 x̄: 27.87 x̃: 30
helped stats (rel) min: 0.08% max: 5.73% x̄: 2.98% x̃: 2.67%
95% mean confidence interval for cycles value: -29.19 -26.54
95% mean confidence interval for cycles %-change: -3.20% -2.76%
Cycles are helped.

Iron Lake
total instructions in shared programs: 7886515 -> 7886507 (<.01%)
instructions in affected programs: 3016 -> 3008 (-0.27%)
helped: 8
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.25% max: 0.28% x̄: 0.27% x̃: 0.27%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -0.27% -0.26%
Instructions are helped.

total cycles in shared programs: 178100396 -> 178100388 (<.01%)
cycles in affected programs: 156128 -> 156120 (<.01%)
helped: 4
HURT: 4
helped stats (abs) min: 4 max: 4 x̄: 4.00 x̃: 4
helped stats (rel) min: 0.02% max: 0.04% x̄: 0.03% x̃: 0.03%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: <.01% max: 0.01% x̄: <.01% x̃: <.01%
95% mean confidence interval for cycles value: -3.68 1.68
95% mean confidence interval for cycles %-change: -0.03% <.01%
Inconclusive result (value mean confidence interval includes 0).

GM45
total instructions in shared programs: 4857872 -> 4857868 (<.01%)
instructions in affected programs: 1544 -> 1540 (-0.26%)
helped: 4
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.25% max: 0.27% x̄: 0.26% x̃: 0.26%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -0.28% -0.24%
Instructions are helped.

total cycles in shared programs: 122167654 -> 122167662 (<.01%)
cycles in affected programs: 96248 -> 96256 (<.01%)
helped: 0
HURT: 4
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: <.01% max: 0.01% x̄: <.01% x̃: <.01%
95% mean confidence interval for cycles value: 2.00 2.00
95% mean confidence interval for cycles %-change: <.01% 0.02%
Cycles are HURT.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>